This invention relates to radiation curable abrasion resistant coatings for solid substrates which are tintable. The compositions of the present invention contain a quaternary ammonium salt as a tintability enhancing compound. The cured coatings of this invention simultaneously resist abrasion and provide superior tintability.
Plastic materials have been used as substitutes for glass in many applications because of their unique properties such as weight, ease of handling, and ease of formation of articles. Most plastics, however, are soft and scratch quite readily. In order to provide abrasion resistance to plastics, silicone coatings were developed. One premier coating that has found wide acceptance for coating plastics is a thermally cured abrasion resistant coating formed from a mixture of colloidal silica and trialkoxy-functional silanes in a alcohol and water medium. Such coating material is disclosed in U.S. Pat. No. 3,986,997, issued Oct. 19, 1976, and assigned to Dow Corning Corporation, Midland, Mich.
Another Dow Corning patent, U.S. Pat. No. 4,554,354, issued Nov. 19, 1985, rendered the coating composition of U.S. Pat. No. 3,986,997 tintable by replacing the methyl group on the trialkoxy-functional silane with a longer chain organic group. The tintability of such coatings was further improved through the addition of a tintability enhancing compound selected from the group consisting of polyhydroxyl-functional compounds and butylated urea formaldehyde compounds, as disclosed in U.S. Pat. No. 5,013,608, issued May 7, 1991, and assigned to Swedlow, Inc., Garden Grove, Calif. These coatings, however, have two major drawbacks. They must be thermally cured which involves a longer cure time than radiation curing and the high temperature cure environments may damage heat sensitive materials. In addition, adhesion of these thermally cured coatings to polycarbonate type resins is very poor, without the additional step of adding a primer. Thus, it would be desirable to develop a tintable abrasion resistant coating which would adhere to polycarbonate type substrates without the use of a primer and which would be radiation curable.
The incorporation of acrylate monomers into abrasion resistant coating compositions have solved some of the above mentioned problems. Acrylate monomers attack the surface of polycarbonate type resins allowing the coating composition to adhere to the substrate without a primer and allow such compositions to be radiation cured. Acrylate monomers, however, necessitated the replacement of the alkoxy-functional silanes used in the thermally cured compositions with silanes capable of reacting with acrylates such as acryloxy, glycidoxy or and/or vinyl functional silanes. Such compositions are described, for example, in U.S. Pat. Nos. 4,822,828, issued Apr. 18, 1989 and assigned to Hoechst Celanese, Corporation, Somerville, N.J., and U.S. Pat. No. 4,486,504, issued Dec. 4, 1984 and assigned to General Electric Company, Waterford, N.Y. While the cured coating films of these compositions are curable by radiation and adhere to polycarbonate without the need of a primer, they have a major drawback. They are not tintable.
Accordingly, the need exits for a single coating composition that, when radiation cured, can accomplish the objectives of high tintability combined with significant resistance to abrasion.